Effect of Ta irradiation on microstructures and current carrying properties of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–</sub><sub><italic>δ</italic></sub>

Abstract

High-temperature superconducting coated conductors (HTS CCs), which are based on epitaxial growth and biaxial texture technology of REBCO thin films, show excellent physical properties including high transition temperature T c,on, high irreversible field H irr, as well as high critical current density J c. However, YBa2Cu3O7– δ (YBCO) has limited applications in a high magnetic field because of low critical current. Previous studies have shown that building smaller artificial pinning centers (APCs) is an effective way to increase the J c of HTS CCs. Compared with the traditional chemical doping route, the particle irradiation method will result in various types and sizes of defects with uniform distribution by controlling the type, energy and dose of particles and keeping the original chemical ratio of the material. 1.2 µm YBCO superconducting layer was deposited on the LaMnO3 substrate of the samples by metal organic deposition (MOD) method. YBCO thin films were irradiated with 1.9 GeV Ta ions at room temperature and vacuum conditions. The microstructure of irradiated samples was systematically observed using a field emission transmission electron microscope (FETEM). The result shows that Ta ions do produce continuous columnar defects with uniform distribution in the superconducting layer of the YBCO films, and the size of the defects is about 10 nm. The basic superconducting parameters, such as T c,on and J c, were measured using magnetic property measurement system (MPMS). The result shows the T c,on of the pristine sample was 91.3 K. When the irradiation fluence was small, the T c,on of the doped sample was basically unchanged. As the fluence reached 5.0×1010 ions/cm2, the T c,on decreased by 0.6 K. In addition, the superconducting antimagnetic phases did not change much until the fluence exceeded 1.0×108 ions/cm2. After the fluence exceeded 1.0×108 ions/cm2, the superconducting antimagnetic phases increased obviously. They increased with the decrease of temperature as well. The result shows that J c increased obviously with the increase of fluence for the samples. In normal situation, the current carrying properties of the samples irradiated in the low field should decrease compared with that before irradiation, but the J c of the samples increased in the low field because of the increase of superconducting antimagnetic phases in the samples after irradiation. At 30 K and 1 T, the J c increased by 3 times of the original, and at 77 K, the J c also improved by 2.7 times in the same field. To analyze the pinning state in this process, the exponential relationship model of J c∝ H – β was used to fit the data of the samples at 30 K and 77 K. The result shows that the doped samples with 5.0×1010 ions/cm2 had the higher value of H * and β , which suggests the generation of new effective pinning centers due to the Ta ion implantation. The normalized pinning force f p = F p/ F p,max of samples irradiated with different fluence was analyzed as a function of the magnetic field h = H / H max by using Higuchi model. The fitting result shows that Ta ion irradiation does affect the pinning behavior of doped YBCO films. Further study is needed to find out why Ta ions have such effects on the YBCO films.